Process for the preparation of dental restorations

ABSTRACT

The invention relates to a process for preparing dental restorations, wherein a lithium silicate glass ceramic or a lithium silicate glass is used which contains at least 8.5 wt.-% transition metal oxide selected from the group consisting of oxides of yttrium, oxides of transition metals with an atomic number from 41 to 79 and mixtures of these oxides.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 14/465,886, filed Aug. 22, 2014, which is a continuation applicationof U.S. application Ser. No. 13/245,089, filed Sep. 26, 2011, now U.S.Pat. No. 8,865,606, which is a continuation-in-part application of U.S.application Ser. No. 13/079,063, filed Apr. 4, 2011, now U.S. Pat. No.8,759,237, which claims the benefit of European Patent ApplicationSerial No. 10160222.5, filed Apr. 16, 2010 and European PatentApplication Serial No. 10168792.9, filed Jul. 7, 2010, and thisapplication further claims the benefit of European Patent ApplicationSerial No. 11162840.0, filed Apr. 18, 2011 and European PatentApplication Serial No. 11173131.1, filed Jul. 7, 2011, all of which arehereby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to a process for the preparation of dentalrestorations wherein lithium silicate glass ceramics and glasses with ahigh content of an element with a high atomic number are used.

BACKGROUND OF THE INVENTION

Lithium silicate glass ceramics are characterized by very goodmechanical properties, which is why they have been used for a long timein the dental field and primarily for preparing dental crowns and smallbridges. The known lithium silicate glass ceramics usually contain asmain components SiO₂, Li₂O, Al₂O₃, alkali metal oxides such as Na₂O orK₂O and nucleating agents such as P₂O₅. In addition, they can contain asfurther components for example further alkali metal oxides and/oralkaline earth metal oxides and/or ZnO. Glass ceramics are also knownwhich contain small quantities of further metal oxides and in particularcolouring and fluorescent metal oxides.

EP 1 505 041 and U.S. Pat. No. 7,316,740, which is hereby incorporatedby reference in its entirety, describe lithium silicate glass ceramicswhich can additionally contain 0 to 2 wt.-% ZrO₂ as well as 0.5 to 7.5wt.-% and in particular 0.5 to 3.5 wt.-% colouring and fluorescent metaloxides. EP 1 688 398 and U.S. Pat. No. 7,452,836, which is herebyincorporated by reference in its entirety, describe similar lithiumsilicate glass ceramics which are substantially free of ZnO and can alsocontain, in addition to the above-mentioned quantities of colouring andfluorescent metal oxides, 0 to 4 wt.-% ZrO₂, wherein however to achievehigh strengths smaller quantities of from 0 to 2 wt.-% ZrO₂ arepreferred. The glass ceramics are processed into the desired dentalrestorations in particular in the form of lithium metasilicate glassceramics by means of CAD/CAM methods, wherein a subsequent heattreatment effects the conversion of the metasilicate phase to thehigh-strength disilicate phase.

U.S. Pat. No. 6,455,451, which is hereby incorporated by reference inits entirety, relates to lithium disilicate glass ceramics which, inaddition to other components, can also contain transition metal oxides.It is proposed inter alia, in order to increase the refractive index ofthe glass matrix, to add small quantities of heavy elements such as Sr,Y, Nb, Cs, Ba, Ta, Ce, Eu or Tb. Thus, for example, CeO₂ and Tb₄O₇ canbe used in quantities of from 0 to 1 wt.-%, Nb₂O₃ and Ta₂O₅ inquantities of from 0 to 2 wt.-% and ZrO₂ and Y₂O₃ in quantities of from0 to 3 wt.-%. In one embodiment, Ta₂O₅ is said to be able to be presentin a quantity of from 0.5 to 8 wt.-%, even though the specific examplescontain at most 2.02 wt.-% of this oxide.

U.S. Pat. No. 5,176,961 and U.S. Pat. No. 5,219,799, which are herebyincorporated by reference in their entirety, disclose glass ceramics forexample for the production of crockery, which can contain as colorantsspecific transition metal oxides such as CeO₂, Co₃O₄, Cr₂O₃, CuO, Fe₂O₃,MnO₂, NiO and V₂O₅ in a quantity of from 0.01 to 7 wt.-%.

U.S. Pat. No. 5,507,981 and U.S. Pat. No. 5,702,514 which are herebyincorporated by reference in their entirety, describe processes forshaping dental restorations and glass ceramics that can be used in theseprocesses. These are in particular lithium disilicate glass ceramicswhich can contain 0 to 5 wt.-% colouring oxides such as SnO₂, MnO, CeO,Fe₂O₃, Ni₂O, V₂O₃, Cr₂O₃ or TiO₂.

Known glass ceramics based on lithium silicate often have opticalproperties which do not adequately satisfy the aesthetic requirements inparticular in connection with the use as dental materials. Thus knownglass ceramics often have an unfavourable refractive index. With glassceramics in particular there is the problem that the refractive indicesof the crystalline phase and of the glass phase usually differ markedlyfrom each other, which in most cases results in an undesired clouding ofthe glass ceramic. Similar problems exist for example in the case ofcomposites because the refractive indices of known glass ceramics andglasses usually differ from those of the polymer phase. There istherefore a need for glass ceramics based on lithium silicate therefractive index of which can be easily varied, but without the otherproperties being substantially impaired. Moreover, it is desirable thatsuch glass ceramics can be prepared and crystallized under conditionscomparable to those for customary glass ceramics and that they canadvantageously be processed to dental restorations, such as inlays orcrowns.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the invention to provide processes for preparingdental restorations wherein glass ceramics and/or glasses can be usedwhich avoid the aforementioned disadvantages and which canadvantageously be processed to the desired restorations.

This object is achieved by the process according to the claims, whichare hereby incorporated by reference. Also, an embodiment of theinvention is the use according to the claims.

The process according to the invention for preparing dental restorationsis characterized in that a lithium silicate glass ceramic or a lithiumsilicate glass is shaped to the dental restoration by

-   -   (i) pressing or    -   (ii) machining,        wherein the glass ceramic and the glass comprise at least 8.5        wt.-% transition metal oxide selected from the group consisting        of oxides of yttrium, oxides of transition metals with an atomic        number from 41 to 79 and mixtures of these oxides.

The prepared dental restorations are preferably inlays, onlays, crowns,veneers, facets and abutments.

Preferably such dental restorations are excluded which are a compositewherein the lithium silicate glass ceramic or the lithium silicate glassis coated on zirconium oxide ceramic. Particularly preferably suchdental restorations are excluded which comprise such a composite.

The dental restoration is preferably on the basis of lithium disilicateglass ceramic as it has excellent optical and mechanical properties.

The glass ceramic and the glass are preferably used in the processaccording to the invention in the form of blanks, e.g. blocks orcylinders.

The pressing or the machining of the glass ceramic or the glass bringthem in the form of the desired dental restoration. Preferably thepressing does not comprise the pressing on another material, such aszirconium oxide ceramic. Such a pressing serves to coat anothermaterial.

The pressing is usually conducted at elevated temperature and elevatedpressure. It is preferred to conduct the pressing at a temperature of700 to 1200° C. Further, it is preferred that the pressing is conductedat a pressure of 2 to 10 bar. During the pressing a change of the shapeoccurs by viscous flow of the material used.

The machining is usually conducted by material-removing processes and inparticular by grinding and/or milling. It is particularly preferred thatthe machining is effected in a CAD/CAM process. Further, it is preferredto conduct at least one heat treatment after the machining in order toconvert shaped glass or shaped glass ceramic into lithium disilicateglass ceramic.

The lithium silicate glass ceramic used in the invention ischaracterized in that it comprises at least 8.5 wt.-% transition metaloxide selected from the group consisting of oxides of yttrium, oxides oftransition metals with an atomic number from 41 to 79 and mixtures ofthese oxides.

In general it is preferred that the transition metal oxide as componentof the glass ceramic used in the invention or of the glass used in theinvention effects substantially no colour change compared with acorresponding glass ceramic or a corresponding glass without theaddition of this component. In particular, the transition metal oxide iscolourless and/or non-fluorescent.

The transition metal oxide is preferably selected from the groupconsisting of oxides of Y, Nb, La, Ta, W and mixtures of these oxides.

Glass ceramics are preferred which comprise 8.5 to 30.0 wt.-%,preferably 9.0 to 25.0 wt.-%, in particular 9.5 to 20.0 wt.-%, preferred10.0 to 18.0 wt.-%, more preferred 10.5 to 16.0 wt.-% and most preferred11.0 to 15.0 wt.-% transition metal oxide selected from one or more ofthe above-named groups.

Surprisingly, by using the high content according to the invention oftransition metal with a high atomic number, the refractive index ofglass ceramics and glasses based on lithium silicate can be easilyadjusted without other properties being substantially impaired. Inparticular it was shown unexpectedly that the high content of transitionmetal with a high atomic number usually neither impedes the desiredcrystallization of lithium disilicate nor leads to the formation ofundesired secondary crystal phases, with the result that glass ceramicswith excellent optical and mechanical properties are obtained accordingto the invention.

A glass ceramic which comprises 54.0 to 80.0 and in particular 60.0 to70.0 wt.-% SiO₂ is further preferred.

In addition, a glass ceramic which comprises 11.0 to 19.0 and inparticular 12.0 to 15.0 wt.-% Li₂O is preferred.

It has proven particularly preferable if the glass ceramic comprises 0.5to 12.0 and in particular 2.5 to 6.0 wt.-% nucleating agents. Preferrednucleating agents are selected from P₂O₅, TiO₂, metals, e.g. Pt, Pd, Au,Ag, or mixtures thereof. Particularly preferably, the glass ceramiccomprises P₂O₅ as nucleating agent. Surprisingly, in particular P₂O₅ asnucleating agent effects the formation of desired lithium disilicatecrystals while largely preventing the formation of undesired secondarycrystal phases.

The glass ceramic used in the invention preferably comprises a furtheralkali metal oxide in an amount of from 0.5 to 13.0, preferably 1.0 to7.0 and particularly preferably 2.0 to 5.0 wt.-%. The term “furtheralkali metal oxide” refers to alkali metal oxide with the exception ofLi₂O. The further alkali metal oxide is in particular K₂O, Cs₂O and/orRb₂O and is particularly preferably K₂O. It is assumed that the use ofK₂O contributes to the strengthening of the glass network compared withthe Na₂O used in conventional glass ceramics. It is preferred that theglass ceramic comprises less than 2.0, in particular less than 1.0,preferably less than 0.5 wt.-% and particularly preferably essentiallyno Na₂O.

It is further preferred that the glass ceramic comprises up to 6.0 wt.-%and in particular 0.1 to 5.0 wt.-% alkaline earth metal oxide, whereinthe alkaline earth metal oxide is in particular CaO, BaO, MgO, SrO or amixture thereof.

It is furthermore preferred that the glass ceramic comprises up to 6.0wt.-% and in particular 0.1 to 5.0 wt.-% ZnO.

The glass ceramic used in the invention can moreover also compriseadditional components which are selected in particular from oxides oftrivalent elements, further oxides of tetravalent elements, furtheroxides of pentavalent elements, melt accelerators, colorants andfluorescent agents.

A glass ceramic which comprises 0.2 to 8.0, in particular 1.0 to 7.0 andpreferably 2.5 to 3.5 wt.-% oxide of trivalent elements is preferred,wherein this oxide is selected in particular from Al₂O₃, Bi₂O₃ andmixtures thereof, and preferably is Al₂O₃.

The term “further oxides of tetravalent elements” refers to oxides oftetravalent elements with the exception of SiO₂. Examples of furtheroxides of tetravalent elements are ZrO₂, SnO₂ and GeO₂, and inparticular ZrO₂.

The term “further oxides of pentavalent elements” refers to oxides ofpentavalent elements with the exception of P₂O₅. An example of a furtheroxide of pentavalent elements is Bi₂O₅.

A glass ceramic which comprises at least one further oxide oftetravalent elements or one further oxide of pentavalent elements ispreferred.

Examples of melt accelerators are fluorides.

Examples of colorants and fluorescent agents are chromophoric orfluorescent oxides of d and f elements, such as the oxides of Sc, Ti,Mn, Fe, Ag, Ce, Pr, Tb, Er and Yb, in particular Ti, Mn, Fe, Ag, Ce, Pr,Tb and Er.

A glass ceramic which comprises at least one and preferably all of thefollowing components is particularly preferred:

Component wt.-% SiO₂ 54.0 to 80.0, in particular 60.0 to 70.0 Li₂O 11.0to 19.0, in particular 12.0 to 15.0 K₂O 0.5 to 13.5, in particular 1.0to 7.0 Al₂O₃ 0.2 to 8.0, in particular 1.0 to 7.0 Alkaline earth oxide 0to 6.0, in particular 0.1 to 5.0 ZnO 0 to 6.0, in particular 0.1 to 5.0Transition metal oxide 8.5 to 30.0, in particular 9.0 to 25.0 P₂O₅ 0.5to 12.0, in particular 2.5 to 6.0 ZrO₂ 0.1 to 4.0, in particular 0.5 to2.0 Colorants and fluorescent agents 0.1 to 8.0, in particular 0.2 to2.0.

The term “main crystal phase” used below refers to the crystal phasewhich has the highest proportion by volume compared with other crystalphases.

The glass ceramic used in the invention preferably has lithiummetasilicate as a main crystal phase. In particular the glass ceramiccomprises more than 5 vol.-%, preferably more than 10 vol.-% andparticularly preferably more than 15 vol.-% of lithium metasilicatecrystals, relative to the total glass ceramic.

In a further preferred embodiment, the glass ceramic has lithiumdisilicate as main crystal phase. In particular the glass ceramiccomprises more than 5 vol.-%, preferably more than 10 vol.-% andparticularly preferably more than 15 vol.-% of lithium disilicatecrystals, relative to the total glass ceramic.

The lithium disilicate glass ceramic used in the invention ischaracterized by particularly good mechanical properties and can beproduced by heat treatment of the lithium metasilicate glass ceramicused in the invention. This can also be effected by pressing inaccordance with the invention at elevated temperature to the desireddental restoration.

It is also surprising that, despite its high content of a transitionmetal with a high atomic number, the lithium disilicate glass ceramicused in the invention usually has a good translucency and noamorphous-amorphous phase separation occurs in it.

The lithium disilicate glass ceramic used in the invention has apartfrom good mechanical properties also a high chemical resistance.

In the process according to the invention also a lithium silicate glasscan be used, which comprises the components of the glass ceramic used inthe invention described above. In respect of preferred embodiments ofthis glass, reference is made to the preferred embodiments describedabove of the glass ceramic used in the invention. It was shownsurprisingly that, despite the high content of transition metal with ahigh atomic number, homogeneous, clear glasses can be obtained whichdisplay no undesired phenomena such as amorphous-amorphous phaseseparation or spontaneous crystallization. These glasses are suitablefor the preparation of the glass ceramic used in the invention by heattreatment.

A lithium silicate glass with nuclei which are suitable for theformation of lithium metasilicate and/or lithium disilicate crystals isparticularly preferred.

The glass used in the invention with nuclei can be produced by heattreatment of a correspondingly composed starting glass. By a furtherheat treatment the lithium metasilicate glass ceramic according to theinvention can then be formed, which in turn can be converted into thelithium disilicate glass ceramic according to the invention by furtherheat treatment. The starting glass, the glass with nuclei and thelithium metasilicate glass ceramic can consequently be seen asprecursors for the production of the high-strength lithium disilicateglass ceramic. The heat treatments required for the conversion can alsobe effected during the pressing at elevated temperature. This wouldresult in the pressing to not only effect the shaping of the glass orglass ceramic used to the desired dental restoration, but to alsoconvert them, e.g. conversion of glass with nuclei to glass ceramic orconversion of lithium metasilicate glass ceramic to lithium disilicateglass ceramic.

The glasses and glass ceramics used in the invention are usually in theform of blanks, as they can easily be further processed in this form.

The process for the preparation of the glass ceramic used in theinvention and the glass with nuclei used in the invention ischaracterized in that a starting glass with the components of the glassceramic or the glass is subjected to at least one heat treatment in therange of from 450 to 950° C.

The starting glass therefore comprises at least 8.5 wt.-% oxide of atleast one transition metal as defined above. In addition, it preferablyalso comprises suitable quantities of SiO₂ and Li₂O, in order to makepossible the formation of a lithium silicate glass ceramic. Furthermore,the starting glass can also contain further components, such as aregiven above for the lithium silicate glass ceramic used in theinvention. Those embodiments are preferred which are also given aspreferred for the glass ceramic.

To prepare the starting glass, the procedure is in particular that amixture of suitable starting materials, such as carbonates, oxides,phosphates and fluorides, is melted at temperatures of in particularfrom 1300 to 1600° C., preferably 1450 to 1500° C., for 2 to 10 h. Toachieve a particularly high homogeneity, the obtained glass melt ispoured into water in order to form a glass granulate, and the obtainedgranulate is then melted again.

The melt can then be poured into moulds to produce blanks of thestarting glass, so-called solid glass blanks or monolithic blanks. Thecooling preferably takes place from a temperature of 500° C. with acooling rate of 3 to 5 K/min to room temperature. This is advantageousin particular for the production of stress-free glass products.

It is also possible to put the melt into water again in order to preparea granulate. This granulate can then be pressed, after grinding andoptionally addition of further components, such as colorants andfluorescent agents, to form a blank, a so-called powder green compact.

Finally, the starting glass can also be processed to form a powder aftergranulation.

The starting glass is then subjected, e.g. in the form of a solid glassblank, a powder green compact or in the form of a powder, to at leastone heat treatment in the range of from 450 to 950° C. It is preferredthat a first heat treatment is initially carried out at a temperature inthe range of from 500 to 600° C. to prepare a glass used in theinvention with nuclei which are suitable for forming lithiummetasilicate and/or lithium disilicate crystals. This glass can thenpreferably be subjected to at least one further temperature treatment ata higher temperature and in particular more than 570° C. to effectcrystallization of lithium metasilicate or lithium disilicate.

This at least one heat treatment can also take place during the pressingat elevated temperature in the process according to the invention. It isalso possible by this heat treatment to convert after machining theglass or glass ceramic used and in particular to convert it tohigh-strength lithium disilicate glass ceramic.

Dental restorations, such as inlays, onlays, crowns, veneers, facets orabutments, can be prepared from the glass ceramic used in the inventionand the glass used in the invention. The invention therefore alsorelates to their use for the preparation of dental restorations. In thisconnection it is preferred that the glass ceramic or the glass areshaped to the desired dental restoration by pressing or machining. Thepressing is in particular conducted at elevated pressure, e.g. 2 to 10bar, and elevated temperature, e.g. 700 to 1200° C. In particular theprocess and the pressing furnace disclosed in EP 231 773 can be used. Asuitable furnace is e.g. the Programat EP 5000 of Ivoclar Vivadent AG,Liechtenstein. For the pressing can be used in particular the startingglass of the invention and preferably the glass with nuclei of theinvention, the lithium metasilicate glass ceramic of the invention andthe lithium disilicate glass ceramic of the invention, e.g. in form ofblanks.

The machining is usually conducted in a CAD/CAM process and it employsin particular the lithium metasilicate and lithium disilicate glassceramic of the invention, preferably in form of suitable blanks. Themachining is in particular effected by material-removing processes, e.g.grinding and/or milling.

After preparation of the desirably shaped dental restoration by pressingor machining, it can additionally in particular be heat treated toconvert precursors, e.g. starting glass, glass with nuclei or lithiummetasilicate, to lithium disilicate glass ceramic.

Finally, the glasses and glass ceramics according to the invention canalso be mixed with other glasses and glass ceramics to give dentalmaterials having desirably adjusted properties. Therefore, a glass orglass ceramic comprising the glass according to the invention or theglass ceramic according to the invention represents a further embodimentof the invention. The glass according to the invention or the glassceramic according to the invention can therefore in particular be usedas a main component of an inorganic-inorganic composite or can be usedin combination with a multitude of other glasses and/or glass ceramics.These composites or combinations are preferably used as dentalmaterials. It is particularly preferred to use the composites andcombinations in the form of sintered blanks. Examples of other glassesand glass ceramics for producing inorganic-inorganic composites andmixtures are disclosed in DE 43 14 817, corresponding U.S. Pat. No.5,432,130, DE 44 23 793, corresponding U.S. Pat. No. 5,698,019, DE 44 28839, corresponding U.S. Pat. No. 5,618,763, DE 196 47 739, correspondingU.S. Pat. Nos. 6,342,458, 5,968,856, and 6,514,893, DE 197 25 552 and DE100 31 431, all of which are hereby incorporated by reference. Theseglasses and glass ceramics belong to the silicate, borate, phosphate oraluminosilicate group. Preferred glasses and glass ceramics are of theSiO₂—Al₂O₃—K₂O type (with cubic or tetragonal leucite crystals),SiO₂—B₂O₃—Na₂O type, alkali-silicate type, alkali-zinc-silicate type,silico-phosphate type and/or SiO₂—ZrO₂ type. By mixing such glassesand/or glass ceramics with the glasses and/or glass ceramics accordingto the invention it is for example possible to adjust the thermalcoefficient of expansion in the desired manner in a broad range of 6 to20*10-6*1/K.

The invention is described in further detail below with reference toexamples.

EXAMPLES Examples 1 to 10 Composition and Crystal Phases

A total of 10 glasses and glass ceramics with the composition given inTable I (each in wt.-%) were prepared by melting corresponding startingglasses followed by heat treatment for controlled nucleation andcrystallization.

The starting glasses were firstly melted in a 100 to 200 g scale fromcustomary raw materials at 1400 to 1500° C. and transformed into glassfrits by pouring them into water. These glass frits were then melted asecond time at 1450 to 1550° C. for 1 to 3 h for the homogenization. Theobtained glass melts were poured into pre-heated moulds to produce glassmonoliths. These glass monoliths were transformed into glasses and glassceramics according to the invention by thermal treatment.

The crystal phases obtained after completion of all heat treatments weredetermined by high-temperature X-ray diffraction (HT-XRD) at thetemperatures listed in each case in Table I. Surprisingly, glassceramics with lithium disilicate as main crystal phase were alwaysobtained. Despite the high content of transition metals with a highatomic number, no secondary crystal phases were found with thesetransition metals.

Finally, the refractive indices of the respective glass phases weredetermined using Abbe refractometry (20° C., 589 nm). It was shown thatthe glass ceramics according to the invention have a much higherrefractive index than a comparison glass ceramic.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

TABLE I 1 2 3 4 5 6 7 8 9 10 SiO₂ 67.4 58.4 66.4 63.5 67.0 61.8 66.466.4 61.8 54.5 K₂O 3.7 1.0 2.9 2.8 2.9 1.0 2.9 2.9 1.0 0.5 Li₂O 14.112.1 13.8 13.2 14.4 13.2 13.8 13.8 13.2 11.3 Al₂O₃ 3.2 1.0 2.9 2.5 1.02.9 2.9 1.0 0.5 P₂O₅ 3.1 2.5 4.0 4.0 4.0 5.0 4.0 4.0 5.0 3.2 WO₃ 8.5Nb₂O₅ 10.0 Ta₂O₅ 10.0 La₂O₃ 25.0 10.0 18.0 30.0 Y₂O₃ 14.0 10.0 18.0 CeO₂1.0 Er₂O₃ 0.3 Tb₄O₇ 0.4 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0100.0 100.0 Crystal Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ phase (s) Li₃PO₄ (800°) KAlSiO₄ (780°)Li₂SiO₃ (800°) Li₂SiO₃ LaPO₄ HT-XRD (800° C.) (820°) (800°) (700° C.)(800° C.) Refractive 1.5312 1.5547 1.5553 1.5494 1.5643 1.5403 1.54221.5586 index n_(d)

Example 11 Direct Preparation of Dental Restorations by Hot-Pressing orMachining (CAD/CAM)

(A) Blanks of glass with nuclei

First of all glasses having the composition according to examples 7 and8 were prepared by mixing corresponding raw materials in the form ofoxides and carbonates for 30 min in a Turbola mixer and then melting themixture at 1450° C. for 120 min in a platinum crucible. The melts werepoured into water in order to obtain finely divided glass granulates.These glass granulates were melted again at 1530° C. for 150 min inorder to obtain glass melts with particularly high homogeneity. Thetemperature was reduced to 1500° C. for 30 min and subsequently a)rectangular glass blanks (12.5 mm×14 mm×40 mm) and b) cylindrical glassblanks with a diameter of 12.5 mm were then poured into pre-heated,separable steel moulds or graphite moulds. The obtained rectangularglass blocks or glass cylinders were then heat-treated in the range of500 to 560° C. depending on the composition to produce nuclei forlithium metasilicate and/or lithium disilicate crystals and tostress-relieve the glass.

The obtained blanks with nuclei were processed according to thefollowing alternatives to restorations.

(B) Hot-pressing of glass with nuclei, lithium metasilicate or lithiumdisilicate glass ceramic

i) The glass cylinders with nuclei (A) were processed by hot-pressing ata temperature of 900-950° C. by means of a pressing furnace EP600,Ivoclar Vivadent AG, to give dental restorations, e.g. inlays, onlays,veneers, partial crowns, crowns and facets.

ii) The glass cylinders with nuclei (A) were subjected to a firstcrystallization at 650 to 750° C. for 20 minutes. The heating-up ratewas 15° C. per minute. After this first crystallisation lithiummetasilicate was detected as main crystalline phase. Throughhot-pressing of the lithium metasilicate glass cylinders at a pressingtemperature of 900-950° C. using a pressing furnace EP600, IvoclarVivadent AG, it was possible to produce dental restorations, e.g.inlays, onlays, veneers, partial crowns, crowns and facets. Thehot-pressing converted lithium metasilicate into lithium disilicate.

iii) The glass cylinders with nuclei (A) were subjected to a firstcrystallisation according to ii) subjected to an additional thermaltreatment at 840 to 880° C. for 5 to 30 minutes. The analysis of thecrystal phases showed after this treatment a glass ceramic according tothe invention with lithium disilicate as main crystalline phase. Thecrystallised cylinders obtained after this second crystallisation weresubsequently processed by hot-pressing at a pressing temperature of900-950° C. using a pressing furnace EP600, Ivoclar Vivadent AG, todental restorations, e.g. inlays, onlays, veneers, partial crowns,crowns and facets.

(C) Machining (CAD/CAM process) of lithium metasilicate

The rectangular glass blocks with nuclei (A) were subjected to a firstcrystallisation in accordance with (B) (ii) to effect crystallisation oflithium metasilicate. The produced lithium metasilicate glass ceramicblocks were then machined by CAD/CAM processes, e.g. Sirona, Cerec 2® orCerec 3®, to give dental restorations, e.g. inlays, onlays, veneers,partial crowns, crowns and facets. Subsequently, these restorations weresubjected to a second crystallisation at 840 to 880° C. for 5 minutes to1 hour. The analysis of the crystal phases showed after this treatment aglass ceramic according to the invention with lithium disilicate as maincrystalline phase.

1. Lithium silicate glass ceramic, which comprises at least 14.0 wt.-%transition metal oxide selected from the group consisting of oxides ofyttrium, oxides of transition metals with an atomic number from 41 to 79and mixtures of these oxides and further comprises lithium metasilicateor lithium disilicate as main crystal phase.
 2. Glass ceramic accordingto claim 1, which comprises more than 5 vol.-% of lithium metasilicateor lithium disilicate crystals.
 3. Glass ceramic according to claim 1,which comprises at least 14.0 wt.-% transition metal oxide selected fromthe group consisting of oxides of Y, Nb, La, Ta, W and mixtures of theseoxides.
 4. Glass ceramic according to claim 1, which comprises 54.0 to80.0 wt.-% SiO₂.
 5. Glass ceramic according to claim 1, which comprises11.0 to 19.0 wt.-% Li₂O.
 6. Glass ceramic according to claim 1, whichcomprises 0.5 to 12.0 wt.-% nucleating agent, wherein the nucleatingagent is selected from P₂O₅, TiO₂, and/or metals.
 7. Glass ceramicaccording to claim 1, which comprises further alkali metal oxide in anamount of from 0.5 to 13.5 wt.-%, wherein the further alkali metal oxideis K₂O, Cs₂O and/or Rb₂O.
 8. Glass ceramic according to claim 1, whichcomprises up to 6.0 wt.-% alkaline earth metal oxide, wherein thealkaline earth metal oxide is CaO, BaO, MgO and/or SrO, and/or comprisesup to 6.0 wt.-% ZnO.
 9. Glass ceramic according to claim 1, whichcomprises 0.2 to 8.0 wt.-% oxide of trivalent elements, wherein theoxide of trivalent elements is Al₂O₃ and/or Bi₂O₃.
 10. Glass ceramicaccording to claim 1, which comprises at least one further oxide oftetravalent elements or at least one further oxide of pentavalentelements.
 11. Glass ceramic according to claim 10, wherein the at leastone further oxide of tetravalent elements comprises ZrO₂, SnO₂ or GeO₂.12. Glass ceramic according to claim 10, wherein the at least onefurther oxide of pentavalent elements comprises Bi₂O₅.
 13. Glass ceramicaccording to claim 1, which comprises at least one of the followingcomponents: Component wt.-% SiO₂ 54.0 to 80.0 Li₂O 11.0 to 19.0 Al₂O₃0.2 to 8.0 K₂O 0.5 to 13.5 Alkaline earth oxide 0 to 6.0 ZnO 0 to 6.0Transition metal oxide 14.0 to 30.0 P₂O₅ 0.5 to 12.0 ZrO₂ 0.1 to 4.0Colorant and fluorescent agent 0.1 to 8.0.


14. Glass ceramic according to claim 1, which is present in the form ofa powder, a blank or a dental restoration.
 15. Glass ceramic accordingto claim 1, wherein the refractive index is at least 1.53.
 16. Lithiumsilicate glass, which comprises the components of the glass ceramicaccording to claim 1 and is capable upon heat treatment of forminglithium metasilicate or lithium disilicate crystals as main crystalphase.
 17. Glass according to claim 16, which is present in the form ofa powder, a blank or a dental restoration.
 18. Lithium silicate glass,which comprises the components of the glass ceramic according to claim 1and further comprises nuclei which are suitable for forming lithiummetasilicate or lithium disilicate crystals as main crystal phase. 19.Glass ceramic according to claim 18, wherein the refractive index is atleast 1.53.
 20. Glass according to claim 19, which is present in theform of a powder, a blank or a dental restoration.
 21. Process for thepreparation of the glass ceramic according to claim 1, in which astarting glass with the components of the glass ceramic is subjected toat least one heat treatment in the range of from 450 to 950° C. 22.Process of using the glass ceramic according to claim 1, as a dentalmaterial, wherein the dental material is used for coating dentalrestorations, for the preparation of dental restorations or as filler ininorganic-organic composites.